Large electric mining rope shovels utilize a digging attachment consisting of a stationary boom with a combination handle/dipper structure which mounts on the boom and actively crowds and hoists into a bank in order to dig with and fill the dipper. The dipper is rigidly connected to the handle. After digging through a bank face and filling with material, the dipper is lifted and the upper portion of the shovel is rotated relative to the lower portion of the shovel so that the dipper is positioned above a waiting dump truck. The operator then activates a tripping mechanism that opens a dipper door latch on the bottom of the dipper and allows a door to swing down and open. The contents of the dipper are then dumped into the truck bed.
The heavy dipper door is pivotally mounted on a lower end of the dipper. The conventional mechanical latch mechanism secures the door in its closed position and, when released, allows the door to open under the force of gravity. The conventional latch mechanisms, as shown in FIGS. 2 and 3, typically include a trip wire 1 or cable assembly which has one end adapted for control by a power shovel operator and another end connected to a moveable latch lever 2 which is generally located on the dipper door 3. The latch lever is typically coupled to a slidable rod or latch bar 4 that is selectively engaged in a latch keeper opening 5 (see FIG. 3) in a front wall 6 of the dipper body of the dipper. The dipper door 3 is held closed when the latch bar 4 is within the latch keeper opening 5. The dipper door is caused to open by tripping the trip cable 1 which moves the latch lever 2 which causes the latch bar 4 to slide away from the latch keeper opening 5 and disengage the latch keeper opening 5, whereby the dipper door 3 will open under its own weight plus the weight of any material contained within the dipper body. The latch lever 2 provides significant mechanical leverage to slide and pull the latch bar 4, which is under very high load.
Normally, the door is thereafter closed by swinging the dipper in such a direction so as to cause the dipper door to move by inertia towards its closed position until the latch bar reengages the latch keeper. More particularly, the latch bar is forced away from the dipper front wall by contacting the latch keeper wall with a sloping surface that causes the latch bar to push up, and then the latch bar drops into the slot by gravity, locking the door.
This is a simple device including mainly two bars pulling on each other with dry, sliding friction contact that has worked well for many years; however, with the increase in dipper size over the past few years, its reliability has become compromised because dry sliding friction levels have increased under higher contact loads making behavior less predictable.